We are utilizing a non-conditional polymerase mutant of B-tropic MuLV to study the mechanism of the reverse transcriptase reaction. This mutant synthesizes partial copies of the natural template in vitro. These products are being characterized in order to determine the nature of the enzymatic blockage to completion of the transcription reaction. At the same time, we are attempting to clone this mutant molecularly in order to completely characterize the genetic change which has led to the structural alteration in the polymerase. In addition, other mutants of tropic MuLVs are being constructed. It is hoped that these various classes of mutants will enable us to correlate known genes and gene products with specific viral functions.